1. Field of the Invention
The present invention relates to a vehicle communication device which performs communication between an in-vehicle device and a moving device to enable operation processing, such as unlocking and locking of a door of a vehicle or engine start.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2012-077587 discloses an electronic key system which performs communication between an in-vehicle device and an electronic key (moving device).
In this electronic key system, a wake signal, a signal for RSSI measurement, and a vehicle ID signal are transmitted from the in-vehicle device. In the electronic key, RSSI information is generated from the average value of electric field strength when receiving the RSSI signal, a key ID signal is generated based on an ID stored in a memory in the electronic key, and the RSSI information and the key ID signal are returned to the in-vehicle device side.
In the in-vehicle device, the received RSSI information is compared with a plurality of thresholds, and an area where the electronic key is present is determined according to the threshold which is exceeded. If the area is determined and it is determined that the IDs match each other when receiving the key ID signal, unlocking or locking of the door is performed, or the engine is started.
In a vehicle communication device, called an intelligent key system, a smart key system, or an electronic key system, which performs communication between an in-vehicle device provided in a vehicle and a portable moving device, an RSSI measurement signal is used, whereby an area where the portable moving device is present with respect to the vehicle is determined
However, if noise is superimposed on the RSSI measurement signal, it is not possible to accurately recognize the area where the moving device is present. In recent years, a fraudulent action of artificially generating and transmitting a false RSSI measurement signal and transmitting a response signal from the moving device to the in-vehicle device even though an occupant is away from the vehicle has been performed.
In the invention described in Japanese Unexamined Patent Application Publication No. 2012-077587, an on time and an off time are included in the signal for RSSI measurement transmitted from the in-vehicle device, and the electronic key generates the RSSI information from the average value of receive electric field strength of both of the on time and the off time. If the off time is provided, when noise is superimposed, the average value of receive electric field strength fluctuates according to the strength of noise. In the in-vehicle device, a threshold for detecting noise is set, and if the RSSI information exceeds the threshold, it is determined that there is an influence of noise.
However, in the electronic key system described in Japanese Unexamined Patent Application Publication No. 2012-077587, since the threshold for detecting noise is set empirically, it is not possible to identity a signal for RSSI measurement including an unexpected noise component. For example, when a false signal for RSSI measurement which has large fluctuation in signal strength but has an average value of signal strength having no significant difference from a normal signal is sent due to a fraudulent action, the RSSI information does not exceed the threshold for noise detection; thus, it is determined that normal communication is performed.